Category: Human evolution

As animals get bigger, so do their brains. But the human brain is seven times bigger than that of other similarly sized animals. Our close relative, the chimpanzee, has a brain that’s just twice as big as expected for its size. And the gorilla, which can grow to be three times bigger than us, has a smaller brain than we do.

Many scientists ask why our brains have become so big. But Karina Fonseca-Azevedo and Suzana Herculano-Houzel from the Federal University of Rio de Janeiro have turned that question on its head—they want to know why other apes haven’t evolved bigger brains. (Yes, humans are apes; for this piece, I am using “apes” to mean “apes other than us”).

Their argument is simple: brains demand exceptional amounts of energy. Each gram of brain uses up more energy than each gram of body. And bigger brains, which have more neurons, consume more fuel. On their typical diets of raw foods, great apes can’t afford to fuel more neurons than they already have. To do so, they would need to spend an implausible amount of time on foraging and feeding. An ape can’t evolve a brain as big as a human’s, while still eating like an ape. Their energy budget simply wouldn’t balance.

The Human Genome Project was officially completed in 2003, but our version of the genome is far from truly complete. Scientists are still finishing the last parts, correcting errors in the official sequence, and discovering new genes. These new genes did not go unnoticed because they are useless or insignificant. Some of them may be key players in our evolutionary story.

Two groups led by Evan Eichler and Franck Polleux have found that humans, alone among all animals, have three extra copies of a gene called SRGAP2, which is involved in brain development. The second of these copies, SRGAP2C, is particularly interesting because it affects the development of neurons, and produces features that are distinctively human. It also emerged between 2 and 3 million years ago, during the time when our brains became much bigger.

New Scientist had a great new feature on nine lost treasures that science wants back. I wrote about one of them – the bones of Peking Man.

In September 1941, Hu Chengzhi placed several skulls into two wooden crates. Around him, China was at war with Japan, so he was sending the skulls to the US for safekeeping. They never arrived. Hu was among the last people to see one of the most important palaeontological finds in history. These lost skulls belonged to Homo erectus pekinensis, known as Peking Man.

We humans are often known as “naked apes”. It might seem like a deserved nickname; after all, we lack the lush coats of body hair that chimps, bonobos and gorillas have in abundance. But we are not naked. We actually have the same density of body hair as other apes of our size, but ours is largely fine and colourless rather than thick and dark. We are coated with a layer of short, fine hair, known technically as vellus hair and colloquially as peach fuzz.
Many scientists have speculated about why we humans have lost a thick coat of body hair. But very few of them have offered answers to an equally mysterious question: why have we kept our vellus coat? The fine hairs aren’t very good at preserving body heat, and they don’t make us more or less sexually attractive. They look like the results of a half-hearted evolutionary stab at becoming hairless. Some have suggested that they have no role at all.

But Isabelle Dean and Michael Siva-Jothy from the University of Sheffield have an intriguing possible answer: they think that the vellus hairs help us to spot parasites.

Humans are remarkably fuel-efficient, or at least, our brains are. The lump of tissue inside our skulls is three times larger than that of a chimp, and it needs a lot more energy to run. But for our size, we burn about as much energy as a chimp. We’re no gas-guzzlers, so how did we compensate for the high energy demands of our brains? In 1995, Leslie Aiello and Peter Wheeler proposed an answer – we sacrificed guts for smarts.

The duo suggested that during our evolution, there was a trade-off between the sizes of two energetically expensive organs: our guts and our brain. We moved towards a more energy-rich diet of meat and tubers, and we took a lot of the digestive work away from our bowels by cooking our food before eating it. Our guts can afford to be much smaller than expected for a mammal of our size, and the energy freed up by these shrunken bowels can power our mighty brains.

This attractive and intuitive idea – the so-called “expensive tissue hypothesis” became a popular one. But Ana Navarrete from the University of Zurich thinks she has disproved it.

Tens of thousands of years ago, our ancestors spread across the world, having sex with Neanderthals, Denisovans and other groups of ancient humans as they went. Today, our genes testify to these prehistoric liaisons. Last year, when the Neanderthal genome was finally sequenced, it emerged that everyone outside of African can trace 1 and 4 percent of their DNA from Neanderthals.

The discovery was a vindication for some and a surprise to others. For decades, palaeontologists had fought over different visions of the rise of early humans. Some championed the “Out of Africa” model, which says that all of us descend from a small group of ancestors who came out of Africa, swept the world, and replaced every other group of early humans. The most extreme versions of this model said that these groups never had sex, or at least, never bred successfully. The alternative – the multiregional model – envisages these prehistoric groups as part of a single population that met and mated extensively.

To an extent, these are caricatured versions of the two models, and there are subtler variants of each. Still, early evidence seemed to support the extreme Out of Africa version. When scientists sequenced the mitochondrial genome of Neanderthals (a small secondary set of genes set apart from the main pack), they found no evidence that any of these sequences had invaded the modern human genome. The conclusion: Neanderthals and humans never bred.

The full Neanderthal genome disproved that idea, but it also shifted the question from whether humans had sex with Neanderthals to just how much sex they had. As I mentioned in New Scientist earlier this year, modern humans were spreading into areas where Neanderthals existed. “It doesn’t necessarily take a lot of sex for genes from a resident population to infiltrate the genomes of colonisers. When an incoming group mates with an established one, the genes they pick up quickly rise to prominence as their population grows.”

Now, Mathias Currat from the University of Geneva and Laurent Excoffier from the University of Berne have weighed into the debate. They simulated the spread of modern humans from Africa and their encounters with Neanderthals throughout Europe and Asia, to work out the levels of sex that would have transferred Neanderthal genes to modern genomes at their current level.

The Neanderthals may be extinct, but they live on inside us. Last year, two landmark studies from Svante Paabo and David Reich showed that everyone outside of Africa can trace 1-4 percent of their genomes to Neanderthal ancestors. On top of that, people from the Pacific Islands of Melanesia owe 5-7 percent of their genomes to another group of extinct humans – the Denisovans, known only from a finger bone and a tooth. These ancient groups stand among our ancestors, their legacy embedded in our DNA.

Paabo and Reich’s studies clearly showed that early modern humans must have bred with other ancient groups as they left Africa and swept around the world. But while they proved that Neanderthal and Denisovan genes are still around, they said little about what these genes are doing. Are they random stowaways or did they bestow important adaptations?

The sequencing of the complete Neanderthals genome was one of the highlights of last year, not just because of the technical achievement involved, but because it confirmed something extraordinary about our own ancestry. It showed that everyone outside of Africa can trace around 1-4% of their genes to Neanderthals. Our ancestors must have bred with Neanderthals on their way out of Africa.

Then, later in the year, the same team revealed another ancient genome. This one belonged to a group of people called Denisovans, known only from a single finger bone and a tooth. They too had left genetic heirlooms in modern people. Around 5-7% of the genes of Melanesians (people from Papua New Guinea, Fiji and other Pacific islands) came from the Denisovans.

In this week’s issue of New Scientist, I’ve got a feature that explores our patchwork origins. I looked at what these ancient genomes mean for our understanding of human evolution. I also considered some intriguing questions like whether other Denisovan fossils have already been found, whether this human pattern is applicable to other animal species, how much you can tell from modern genomes alone, and whether we’ll ever get DNA from the ‘hobbits’ of Flores. Do check it out – it contains some great viewpoints from Svante Paabo and David Reich, two of the scientists who spearheaded the sequencing efforts, along with Chris Stringer, Milford Wolpoff, Alan Cooper and John Hawks.

The magazine’s on the stands for the next week, or you can read the piece online if you have a New Scientist subscription to read the full thing. If get round to it, I’ll try and stick up some of the transcripts from the interviews that I did for the piece. There’s some great stuff there.

Many birds have a compass in their eyes. Their retinas are loaded with a protein called cryptochrome, which is sensitive to the Earth’s magnetic fields. It’s possible that the birds can literally see these fields, overlaid on top of their normal vision. This remarkable sense allows them to keep their bearings when no other landmarks are visible.

But cryptochrome isn’t unique to birds – it’s an ancient protein with versions in all branches of life. In most cases, these proteins control daily rhythms. Humans, for example, have two cryptochromes – CRY1 and CRY2 – which help to control our body clocks. But Lauren Foley from the University of Massachusetts Medical School has found that CRY2 can double as a magnetic sensor.

For every hundred babies born in Europe, one or two of them are quickly followed by a twin. From a cold evolutionary point of view, twins look like a win for their mother. History, after all, is written by individuals who are best at passing on their genes, so having more children at once seems like a good strategy. So why are twins so rare?

The standard answer is that giving birth to twins, and raising them, is difficult. Complications during pregnancy and delivery could kill both mother and children. In a classic 1990 paper, a bird specialist called David Anderson expanded on this idea. He suggested that twins are the result of an evolutionary bet-hedging strategy gone wrong.

Anderson knew that birds will commonly lay several eggs as insurance, to make sure that they get at least one strong chick. After hatching, the parents (or the strongest chick) will kill the weaker babies. In humans, a similar (but less brutal) competition happens in the womb. Mothers will often produce several eggs, with the others acting as insurance in case the first doesn’t make it. If two eggs are fertilised, one embryo is often lost – this is why many more twins are conceived than are born, the so-called “vanishing twin” effect. If this process fails, you get twins. That’s a problem, because having twins exacts a physical toll upon the mother.

According to this fairly bleak view, mums with twins have taken an evolutionary gamble that has backfired. But Shannen Robson and Ken Smith don’t agree. In a new study, the two scientists from the University of Utah have painted the birth of twins in a more positive light. To them, the very fact that some mothers can bring twins to term is a sign that they are strong and fit. The presence of twins singles out mums who can bear the extra cost of having twins.